Seems the predicates that you were trying to import in the schema had some pending transactions when you started the EC2 instance. If there are pending transactions on the predicates in schema, then Dgraph schema can’t be altered, although, the GraphQL schema was saved. This results in an inconsistent state between the GraphQL and Dgraph schema. One should just retry updating the schema after some time in this case. And, as you noticed, it worked for you after some time. So, this one is not really a bug.
But, this error message may also occur when the GraphQL layer has generated a wrong Dgraph schema. And, as that would be a bug, that’s why the message says so 
. I guess we can improve the error message to say that this may be a bug and not necessarily a bug. Depends on what the old and new schema’s were.
We have mentioned the GitHub URL in those error messages which if encountered mean a bug, and should be reported. I guess we can update them to point to Discuss now.